1. Field of the Invention
The invention relates generally to the field of aircraft fuel tank access doors. More specifically, the present invention relates to fuel tank access doors having features that protect against lightning, precipitation static, and corrosion.
2. Description of the Related Art
Aircraft wings are equipped on their undersides with fuel tank access doors that allow for inspection of fuel tanks located within the wings. Generally, each access door includes an inner door located inside the wing and an outer door located outside the wing. The inner and outer doors may be secured to the wing by clamping a portion of the aircraft skin between the doors, and fastening the doors directly to one another.
Typically, the outer fuel tank access door is made of aluminum to meet Federal Aviation Administration (FAA) fuel pooling fire requirements, and lightning protection is provided by purposeful arcing between the outer fuel tank access door and the aircraft skin. This arcing is accomplished by use of a high impedance gasket between the door surface and the aircraft skin land area. In order to also protect against precipitation static (“p-static”) the high impedance gasket is typically impregnated with carbon material and the land area is cleaned and painted with a p-static paint. This design works well for p-static; however, it results in galvanic action causing corrosion between the wing and access door surfaces.
A number of patents have disclosed components for providing lightning protection for an aircraft. For example, U.S. Pat. No. 4,291,816 discloses inner and outer door panels made of bendable material, where an electrically conductive surface, such as aluminum foil, may be adhered against the outer door panel to provide electrical conduction to the panel; U.S. Pat. No. 4,352,142 discloses a composite sandwich structure of aluminum and dielectric material applied to the surface of an aircraft structure to confine damage from a lightning strike to the sandwich layer which can be removed and replaced; U.S. Pat. No. 4,630,168 discloses a fastener having a metallic body and a dielectric cap covering a top portion of the fastener head, which allows the countersunk fastener to be painted along with the aircraft skin; and U.S. Pat. No. 7,050,286 discloses a structure for directing electrical dissipation from a lightning strike through the aircraft skin to prevent arcing at a fastener.